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>50.1. Foreign Data Wrapper Functions</A
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><P
>     The FDW author needs to implement a handler function, and optionally
     a validator function. Both functions must be written in a compiled
     language such as C, using the version-1 interface.
     For details on C language calling conventions and dynamic loading,
     see <A
HREF="xfunc-c.html"
>Section 35.9</A
>.
    </P
><P
>     The handler function simply returns a struct of function pointers to
     callback functions that will be called by the planner and executor.
     Most of the effort in writing an FDW is in implementing these callback
     functions.
     The handler function must be registered with
     <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> as taking no arguments and
     returning the special pseudo-type <TT
CLASS="TYPE"
>fdw_handler</TT
>.  The
     callback functions are plain C functions and are not visible or
     callable at the SQL level.  The callback functions are described in
     <A
HREF="fdw-callbacks.html"
>Section 50.2</A
>.
    </P
><P
>     The validator function is responsible for validating options given in
     <TT
CLASS="COMMAND"
>CREATE</TT
> and <TT
CLASS="COMMAND"
>ALTER</TT
> commands for its
     foreign data wrapper, as well as foreign servers, user mappings, and
     foreign tables using the wrapper.
     The validator function must be registered as taking two arguments, a
     text array containing the options to be validated, and an OID
     representing the type of object the options are associated with (in
     the form of the OID of the system catalog the object would be stored
     in, either
     <TT
CLASS="LITERAL"
>ForeignDataWrapperRelationId</TT
>,
     <TT
CLASS="LITERAL"
>ForeignServerRelationId</TT
>,
     <TT
CLASS="LITERAL"
>UserMappingRelationId</TT
>,
     or <TT
CLASS="LITERAL"
>ForeignTableRelationId</TT
>).
     If no validator function is supplied, options are not checked at object
     creation time or object alteration time.
    </P
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